Earlier, it was believed that amyloid resulted from the extrusion of some plasma constituent into tissues, in a manner similar to the formation of fibrin from fibrinogen in plasma. Later, it was thought to be caused by reactions between antigens and antibodies that were deposited in tissues instead of being removed. 

Recent studies suggest that the deposition of abnormal proteins as amyloid may be caused by a number of different factors, including the presence of specific genes. Amyloid is not a single substance, nor does it have a single cause.